1. Field of the Invention
The present invention relates to an optical system for a liquid crystal projector, and more particularly, to an illumination system in a liquid crystal projector.
2. Background of the Related Art
Recently, projectors are paid attention as flat displays that are thin and have a large sized screen. As the projector, which enlarges and projects a small picture, a liquid crystal projector that is thin is used mostly. However, the liquid crystal projector can not provide a clear picture under a bright environment. In order to solve this problem, a related art projector employs lenses in an illumination system. FIG. 1 illustrates a related art illumination system in a liquid crystal projector.
Referring to FIG. 1, the related art illumination system is provided with first, and second fly eye lenses 4 and 6, and a polarizing beam sprite (PBS) array, which are arranged in parallel on an optical path between a light source 2 and a focusing lens 9.
A beam of white light from the light source 2 is incident on the first fly eye lens 4, and the first fly eye lens 4 having a plurality of micro-lens cells splits the beam in lens cell units, and directs to respective lens cells of the second fly eye lens 6. The second fly eye lens 6 refracts the beam from the first fly eye lens 4 into a parallel beam, and directs to the PBS array 8, and the PBS array 8 splits the beam from the second fly eye lens 6 into a P polarization beam and an S polarization beam. The PBS array 8 forwards the split S polarization beam as it is while the PBS array 8 converts the P polarization beam into an S polarization beam before forwarding the P polarization beam. The focusing lens 9 converges the S polarization beam from the PBS array 8 to a minimum incidence angle for preventing an optical loss and improving an optical efficiency. A luminance efficiency of the foregoing illumination system in the liquid crystal projector is related to correspondence of the beam from the light source with the lens cells of the first fly eye lens.
Referring to FIG. 2, a beam of light emitted from a lamp 2A is totally reflected at a parabolic reflector 2B, and incident on a first fly eye lens 4 in a form of quasi-parallel beam. In this instance, while most of the beam from a center ‘P’ of the lamp 2A is reflected at the parabolic reflector 2B, and incident on a center point of respective lens cells of the first fly eye lens 4, the beams L1, and L1′ reflected in the vicinity of a center part ‘0’ of the parabolic reflector 2B are not incident on center points of the lens cells 4A and 4B, but on points deviated to one side of the center points.
That is, referring to FIG. 3, it can be known that the beams of lights L1, L2, and L3 emitted from the lamp 2A, and reflected at a first reflection point RI in the vicinity of the center part ‘0’ the parabolic reflector 2B are incident on a first lens cell 4A in the center part of the first fly eye lens 4. In this instance, the first beam L1 emitted from a center point P2 of the lamp 2A, and reflected at the first reflection point R1 of the parabolic reflector 2B is, not incident on a center of the first lens cell 4A, but a point deviated to one side of the center point, because a first angle θ− and a second angle θ+ differ owing to a length of the lamp 2A. The first angle θ− is an angle between the first beam L1 and the second beam L2 incident on the first reflection point R1 on the parabolic reflector 2B from the first point P1 and the second point P2 of the lamp 2A respectively, and the second angle θ+ is an angle between the third beam L3 and the first beam L1 incident on the first reflection point R1 on the parabolic reflector 2B from the third point P3 and the first point P1 of the lamp 2A respectively. The first, and the second angles may be expressed as the following equations (1).       θ    -=                  tan        ⁢                                   ⁢                  1                      -            1                          ⁢                              -            z1                    y1                    -              tan        ⁢                                   ⁢                  1                      -            1                          ⁢                                            -              z1                        +                          L              /              2                                y1                      ,          ⁢      θ    +=                  tan        ⁢                                   ⁢                  1                      -            1                          ⁢                                                            -                L                            /              2                        -            z1                    y1                    +              tan        ⁢                                   ⁢                  1                      -            1                          ⁢                              -            z1                    y1                      ,
Where, y1 and z1 denote coordinates of the first reflection point R1 on z-y coordinate axes in FIG. 2, and ‘L’ denotes the length of the lamp 2A. It can be known from the equation (1) that the first, and second angles θ− and θ+ differ, wherein the second angle θ+ is smaller than the first angle θ+, relatively.
Thus, due to the difference of the first, and second angles, the second beam L2 from the second point P2 of the lamp 2B is reflected at the first reflection point R1 on the parabolic reflector 2B, and incident on a second lens point FL2 at a top of the first lens cell 4A, the third beam L3 from the third point P3 of the lamp 2B is incident on a third lens point FL3 at a bottom of the first lens cell 4A via the first reflection point R2, and the first beam L1 from the first point P1 is incident on a first lens point FL1 at a location deviated from a center of the first lens cell 4A to downward via the first reflection point R1. In other words, an arc light emission center point P1 of the lamp 2A and the center point of the lens cell 4A are not in correspondence. As shown in FIG. 2, such a non-correspondence of the center points mostly occurs at lens cells 4A and 4B in a central region of the first fly eye lens 4.
FIG. 4 illustrates a beam distribution of an image formed on a second fly eye lens 6 through a first fly eye lens 4 of beams from the lamp 2A, wherein it can be known that beam distributions on lens cells in a central part ‘A’ are scattered widely in a height direction of the second fly eye lens 6 more than beam distributions on lens cells in other parts of the second fly eye lens 6. An optical efficiency in the center part of the second fly eye lens 6 where the beam distribution is greater is poor in comparison to the other part when the beam distribution is smaller. Accordingly, the non-correspondence of the center points of the lamp and the lens cells is a cause of the poor optical efficiency of the liquid crystal projector.